1. Field of the Invention
The present invention relates to an improved airtight seal, with magnetic bodies acting on two elements, to create a contact-seal inside, in combination with two labyrinth-seal type on each end. The configuration results in a compact single body of easy installation in centrifugal pumps, within the annulus between the bearing casing and the rotatable shaft.
2. Description of the Related Art
The oil industry and other diverse processes with great frequency involve, and make use of, various types of centrifugal pumps.
Petroleum, from its extraction on production platforms, to its transportation through various methods, such as oil pipes or tanker ships, and mainly in the refining process itself, in specific industrial parks, needs to be pumped through thousands of meters of pipes, hundreds of systems, which, in the popular jargon, are known as the oil extraction and refining process.
Thus, the most important components and those most subjected to wear in the oil industry, are the pumps, which are used in diverse applications, which come in various models and powers, which require great attention in their specifications and maintenance.
Due to the fact that they are an indispensable component in any stage of the oil industry, many pumps are used without interruption, stopping only in the event of failures or for scheduled maintenance.
However pumps have ball bearings, inside the bearing casings, that are sensitive to any contaminant from the environment where the pump is running. They also possess a component particularly subject to failure, the seal, which isolates the ball bearings on the inside of bearing casings as well as preventing any trace of lubricant inside the bearing casing from contaminating the environment.
In this sense, there are some models of seals, which for the purpose of being in accordance with standards, try to satisfy to the maximum these conditions of isolation. One of these is the American Petroleum Institute Standard—API 610,9th ed. (2003), which states:
“5.10.2. 7—Bearing cases must be designed to prevent contamination from moisture, dirt and other foreign objects . . . This must be accomplished not by injecting compressed air, but through the use of seals, such as labyrinth or magnetic seals, in the areas where the shaft crosses through the casing”
However, complete fulfillment of this requirement can only be accomplished by using a contact seal, therefore using labyrinth seals, even though they are accepted by the standard, allows steam to enter and condense inside the oil. This option, in practice, generates great losses to the oil industry, especially when all the hours involved in down time is accounted for in the production process, as well as the cost of man-hours in maintenance and repair of the ball bearings in the pumps.
It has been possible to measure these damages due to studies that have demonstrated that the presence of only 0.02% of water in the oil of casings is sufficient to reduce the useful life of the ball bearings up to 48%.
Thus, the greatest concern of the technician responsible for maintenance of the pumps is to make the best choice from the existing models of seals, in order to prevent failures, especially when dealing with 20 centrifugal pumps. Any unscheduled downtime, especially those caused by failures of the mechanical seal in the bearing casings, not only interrupts the process with which that pump is associated, but, invariably, damages many other pump components as well.
Therefore, the better the quality of the isolation provided by the casing seal, the greater the durability of the equipment. It is in this way that the oil industry's technical designers attempt to surpass the requirements of Standard API 610,9th ed. (2003), and they conduct their projects so that the seal of the bearing casings (mainly in centrifugal pumps), will be completely air tight.
Currently, there are some models known that attempt to fulfill this condition, for example:
1) Mechanical Airtight Seal That Uses Springs.
The GBS® model developed by the A. W. Chesterton Company is a good example. This type of airtight seal has a good service life expectancy and performance when mounted correctly, however the assembly requires special attention be given to its alignment. It is a seal that occupies little external space, and offers a descending wear curve. However when the required criteria are not observed when aligning the seal, many failures will occur. The seal assembly is difficult due to the high degree of interference presented for the O-ring in the rotary head. The repair of the seal is difficult and offers little protection against direct jets of liquid on the outside surface of the operation.
2) Magnetic Airtight Seal, With Magnets in the Attractive Position.
The Magnum-S® model may be mentioned as an example developed by the Isomag Corporation. This type of airtight seal is easy to assemble and occupies little external space. However it offers an upward wear curve, because the greater the wear of the material, the greater the distance between magnets and greater the force of attraction, causing greater wear and a shorter service life expectancy. It is also a seal that cannot be repaired and offers little protection against direct jets of liquid on the outside surface of the operation.
3) Magnetic Airtight Seal, With Magnets in the Repulsive Position.
The RMS 700® model may be mentioned as an example developed by the Improseal Corporation. This type of airtight seal needs a great amount of free external area, does not offer any type of external protection, which is totally exposed to the inclemency of the environment. This also is a seal that cannot be repaired. Its characteristic of being exposed to dirt and because it requires a large external mounting area, makes it practically impossible to adapt it to pumps (mainly centrifugal pumps).
In the case of the bearing casing seal models that currently use magnets in the repulsive position specifically; their construction is that of a stationery head.
The head is one of the key elements for maintaining the impermeability of the project, with its function being to support the primary seal, which is pressed against its surface in such a way as to maintain permanent and hermetic contact.
This contact between the head and the primary seal must be preserved from contamination emanating from the environment, such as dust, solid debris, or direct jets of liquids, as it prevents in this way premature eroding between the head and the primary seal, which is already in a permanent state of friction.
The head is fixed upon the sleeve by means of an O-ring and this set of components travels along with the rotation of the shaft.
The primary seal is fixed to the housing by means of an O-ring, behind which are also mounted two collars of magnets in the repulsive position. The magnets in direct contact with the housing and the back of the primary seal travel along with the rotation of the shaft, and promote the compression of the primary seal towards the rotary head.
There is a great advantage to this type of seal created through pressure caused by the repulsive force of magnets: with the passage of time, wear due to friction between the head and the primary seal will cause aberrance between the magnets pressuring the seal against the head. This aberrance will result in a reduction in the repulsive force, and consequently in the friction pressure between the head and the primary seal. This results in less abrasion between these two components and, for this reason, a much greater service life expectancy and preservation of the properties of impermeability. However the seals on bearing casings that use this sealing system, present problems with alignment that can invalidate this advantage.
Seals, such as those mentioned in the 1st example, that use rotary heads may offer several advantages which may be invalidated if not mounted with perfect alignment, therefore, in the event that the alignment is not accomplished, abrasion may be even greater than the other models of seals for bearing casings that use springs or magnets in the attractive position.
Alignment is a requirement that is very difficult to achieve in the bearing casings of models currently in existence, because the head is stationery, and the primary seal is mounted upon a rotary sleeve. In turn, the sleeves are provided with only one O-ring.
This type of construction that places the sleeve on the only O-ring causes a misalignment in the setting of the head on the primary seal, and requires that the repulsive action of the magnets continuously correct, on every turn, the deficiencies of this misalignment.
In models that are currently available on the market, perfect alignment is only obtained by very meticulous installation work and preferably calibrated with a time source dial indicator for centering. It is a task that in addition to requiring its own specialized tools, demands time and must be performed by a skilled technician.
The material currently used on the contact surface of the head presents yet another problem. Generally, stainless steel is used, which is somewhat deficient in its capacity to resist abrasion, and also generates high temperatures.
Therefore, no seal model for bearing casings currently available offers a repair feature for the seal assembly. Due to fact that their most sensitive components are generally susceptible to excessive exposure to inclemency, at the end of their service lives, they are so deteriorated that repair is not feasible.
The three models of airtight seals for bearing casings, in spite of offering advantages, also present disadvantages that may result in premature failures or the need for periodic maintenance.
Considering the fact that each pump needs at least two seals, and that a refinery of average size is using around 1000 pumps, easily proves the problem and the damage caused by the disadvantages present in the current sealing technique used in bearing casings.